This invention relates to the separation of minerals from ore, and, more particularly, to the separation of nahcolite-containing ore particles from oil shale particles in a mixture of particles of nahcolite-bearing oil shale ore.
Large deposits of oil-bearing shale have been discovered in this and other countries, but have remained relatively untapped for want of an economic process for recovery of the oil at a cost comparable to conventional oil sources. The increasing cost of liquid petroleum, however, has sharpened interest in producing oil from oil shale deposits. Some of these oil shale deposits also contain substantial quantities of other valuable minerals, recovery of which will help to make the economics of the oil recovery more attractive.
One of the minerals found in substantial quantities in some oil shale deposits is nahcolite, a naturally occurring sodium bicarbonate, which has been found useful in reducing the sulphur and nitrogen content of hydrocarbons and flue gas. Efficient separation of this nahcolite from the remaining mined oil shale ore will greatly enhance prospects for possible recovery of oil and other minerals from such deposits.
As used herein, the term "nahcolite" refers to the naturally occurring sodium bicarbonate found in oil shale deposits. "Oil shale ore" refers to nahcolite-bearing oil shale which includes nahcolite-containing ore particles and oil shale particles, and "oil shale" refers to the residue which remains after the nahcolite-containing particles have been removed from the oil shale ore.
Processes for separating nahcolite from oil shale are disclosed in U.S. Pat. Nos. 3,962,403 to Wyslousil and 3,992,287 to Rhys. In each of these processes, the mined oil shale ore is subjected to one or more treatment steps to produce a surface color change, thereby enabling optical sorting of the nahcolite from the host oil shale. In U.S. Pat. No. 3,962,403, the color change is accomplished by heating the ore by direct or indirect heat to convert at least some of the surface bicarbonate to a lighter colored carbonate, while the process of U.S. Pat. No. 3,992,287 involves spraying water on the surface of the ore to produce a wetted film of bicarbonate solution which is then subjected to microwave drying to produce a surface film of bicarbonate crystals having a lighter color than the host oil shale. The cost of producing such a color change in the nahcolite-containing portion of the mined oil shale ore, however, represents a significant portion of the overall cost of the nahcolite recovery operation. It is therefore desirable to separate the nahcolite-containing ore particles from the host oil shale particles without the expense of producing the surface color change heretofore required.